Re: What material to stuff inside drywall cavbity?
The biggest effect the insulation is going to have in a typically framed wall is preventing wall cavity resonance. The cavity resonance will be affected more by the amount of fill rather than the "R" value of the fill itself which relates to thermal transmission, the higher the "R" value, the less transmission there is. Insulation will have little effect on sound transmission (assuming typical framing and no penetrations) other than any mass damping affect. For mass damping i would look to a Dynamat type product instead. Also make sure to seal all penetrations, preferably with a fire retardent material.
With respect to carpet pads for your floor, there are a few manufacturers of mass loaded pads that will reduce the noise somewhat, of course, the noise reduction would be greatly enhance if you also isolate the floor although that get expensive, especially for a floor over the garage. (assuming you are trying to keep your theater noise from escaping rather than keeping environmental noise out).